1. a. Field of the Invention
The present invention relates to a wide-angle illuminating optical system with light guide arranged to be capable of illuminating over a wide observing area.
2. B. Description of the Prior Art
In recent years, there is a large tendency for endoscopes to make the observing area large. For this purpose, wide-angle lens systems are developed as optical systems for endoscopes. In known illuminating methods employing a light guide for transmitting the illuminating light such as an optical fiber bundle which have been used for endoscopes, even the theoretical angle covering the illuminating area is 60.degree. and the actual angle covering the effective illuminating area is 50.degree. only. Therefore, even when it is attempted to use an observing objective having a viewing angle exceeding the above-mentioned angle, it is very difficult or impossible to observe the marginal portion of the observing area because such portion is not illuminated satisfactorily or not illuminated at all and, consequently, the wide-angle observing objective cannot display its effect satisfactorily.
To solve the above problem, it is necessary to make the illuminating area of the illuminating light wider so that the above-mentioned wide-angle objective for endoscopes will fully display its effect. For this purpose, it is known to widen the illuminating area by providing a concave lens 2 at the end of the light guide 1 as shown in FIG. 1. This method, however, has the following disadvantages. That is, in endoscopes employing a light guide for illumination, especially in forward-viewing endoscopes of such type, a hood 3 is provided at the distal end of the endoscope in order to prevent the end face of the light guide or front lens surface of the observing objective from closely contacting the surface of the object to be observed. Therefore, in the method illustrated in FIG. 1 in which the field angle of the illuminating optical system is widened by providing a concave lens at the end of the light guide, the illuminating light is partially cut by the hood 3. Moreover, in endoscopes, the end face of the image guide for observation is generally arranged at a position shifted from the position of the end face of the light guide for illumination. Therefore, when the angle covering the illuminating area of the illuminating lens system is about the same as the field angle of the observing lens system, a part of the observing area still becomes dark and, consequently, it is necessary to use an illuminating lens system having a still wider angle.